This invention relates generally to the sampling of analog signals in an electronic instrument, such as protective relays for power systems, and more specifically concerns a sampling system, which includes a unipolar A/D converter.
In conventional microprocessor-based protective relays used for protection of electric power systems, the three separate phase voltages (V1, V2 and V3) and the three separate phase currents (I1, I2 and I3) from the power line being protected are applied through a step-down transformer or other step-down conversion system to reduce the magnitude of the voltage and current values. The stepped-down values are then low-pass filtered and multiplexed prior to the application of those values to an analog-to-digital converter. The analog-to-digital converter converts the analog input signals to digital signals, which are then applied to a conventional processor (microcontroller) which performs selected conventional protection functions using the input signals and provides trip signals for a circuit breaker when the results of the processing indicate a fault on the power line.
Such systems and other electronic instruments use a bipolar analog-to-digital converter, which measures both positive and negative voltages with respect to ground. A bipolar A/D converter is required when it is anticipated that the inputs will be both positive and negative with respect to ground.
However, conventional bipolar A/D converters are relatively expensive. Use of a unipolar A/D converter can reduce the cost of protective relays, which is desirable, since they are less expensive than bipolar A/D converters. While unipolar A/D converters are less expensive than bipolar A/D converters, the signals applied to the input of the converter are required to be completely positive, i.e. the entire signal must be above ground. This has meant modifying the incoming signal in some fashion to make the incoming signals always positive, such as offsetting the ground reference of the step-down stage, or injecting an offset between the step-down stage and the low-pass filter or within the low-pass filter, so that the output from the low-pass filter stage will always be positive with respect to ground.
However, these attempts each have their own respective disadvantages, in some cases involving the gain accuracy of the circuit and in other cases introducing a phase shift and a difference in A/D response, which will differ in accordance with the frequency of the signal. Accordingly, unipolar A/D converters have generally not been used in protective relay applications or other similar instruments.
Accordingly, the present invention is a system for sampling a bipolar analog signal, the sampling system being used in a protective relay for an electric power system, comprising: a unipolar A/D converter for processing bipolar input analog signals in the form of voltage and current values to corresponding digital output signals, wherein the A/D converter has a ground pin reference voltage which is at least as negative as the most negative portion of the input signals to be processed by the A/D converter, such that the entire range of input signals is processed by the A/D converter to produce a corresponding digital output signal.